The present invention relates to a vibration detection system, a signal processing apparatus, and a signal processing method for removing influence of vibration waves generated from a seismic source device and detecting vibrations in the ground.
Conventionally, an active seismic exploration for observing a state in the ground by radiating artificial vibration waves into the around and receiving the vibration transmitted in the ground near the ground surface is known. The Accurately Controlled Routinely Operated Signal System (ACROSS), which is a permanent seismic source device, has been noticed as a stationary seismic source that is suitable for such an active seismic exploration. The ACROSS can generate precisely controlled signals (vibration waves) by rotating an eccentric weight and is suitable for observation in the ground.
Japanese Unexamined Patent Application Publication No. 2007-304100 discloses a method for observing a state in the ground by recording energy of reflected waves that are originated by reflection of signals generated by a seismic source device at a dike layer.
It should be noted that shale gas has recently become a new attractive natural gas resource. Shale gas is collected by using hydraulic fracturing that includes inserting a pipe horizontally into a shale layer containing shale gas and making artificial fractures by injecting high-pressure water through this pipe.
Here, it is important when controlling the hydraulic fracturing to monitor a fracture generating area by continuously observing microseisms caused by the fracturing. When the seismic source device used for the above-mentioned active seismic exploration is operated in the vicinity of a point where the hydraulic fracturing is performed, the vibration waves generated from this seismic source device become noise in the microseisms caused by the hydraulic fracturing and a precise observation of the microseisms cannot be performed. Accordingly, in general cases, the operation of the seismic source device is stopped while the microseisms are observed, and the active seismic exploration is performed at a time other than while the microseisms are observed.
Further, it is important to continuously monitor a foreshock and a preslip for predicting a natural earthquake, but the vibration waves generated from the seismic source device appear as noise in the monitoring of a natural earthquake. Since nobody knows when a natural earthquake occurs, an operation of the seismic source device is not able to be stopped according to the occurrence of a natural earthquake, hence further contrivance that allows monitoring without stopping the seismic source device is required.